Below are some of the most commonly misused homonyms (words that sound the same, but have different spellings and meanings).
EFFECT v. AFFECT:
To affect means to influence. An effect is a result. Example: Antihistamines affect people in different ways; one effect is drowsiness.
ITS v. IT'S:
"Its" is the possessive form of "it." "It's" is a contraction of "it is." Example: The ball lost its bounce. It's flat now.
THAN v. THEN:
Than indicates a comparison. Then indicates a point in time. Example: Rather than call a locksmith after forgetting his keys, Roger then broke the back window to get into his house.
THERE v. THEIR v. THEY'RE:
"There" indicates a place. "Their" is a possessive pronoun. They're is a contraction of "they are." Example: The ecologists are over at the lake. They're compiling their data over there.
YOUR v. YOU'RE:
"Your" is a possessive pronoun. "You're" is a contraction of "you are." Example: You're certain that this your bike?
TO, TOO, TWO:
"To" is a preposition. "Too" is an adverb. "Two" is a number. Example: When you go to Jen's house, bring the fruit salad. And don't forget to bring two serving spoons, too.